1. Field of the Invention
The invention relates to a device for producing extreme UV radiation which emits extreme UV radiation. The invention relates especially to a device for producing extreme UV radiation using tin as the radiation source of the extreme UV radiation in which the service life of the evacuation device and the like is prolonged by producing hydrogen radicals.
2. Description of Related Art
An improvement in resolution has been required in projection exposure tool with miniaturization and increased integration of integrated semiconductor circuits for its manufacture. To meet these requirements, the wavelengths of the exposure radiation source are being increasingly shortened. As a semiconductor exposure radiation source of the next generation in succession to an excimer laser device, a device for producing extreme UV radiation (hereinafter also called an EUV radiation producing device) is being developed which emits extreme UV radiation (hereinafter EUV radiation) with wavelengths from 13 nm to 14 nm, especially with a wavelength of 13.5 nm.
Several schemes are known for producing EUV radiation in an EUV radiation producing device. In one, a high density and high temperature plasma is produced by heating and excitation of an EUV radiating species and EUV radiation is extracted from this plasma.
The EUV radiation producing device adopting such a scheme based on the method of production of a high density and high temperature plasma is roughly divided into an EUV radiation producing device of the LPP (laser produced plasma) type and an EUV radiation producing device of the DPP(discharge produced plasma)type.
In an EUV radiation producing device of the LPP type, EUV radiation emitted from a high density and high temperature plasma which is formed when irradiated targets such as solids, liquid, gas and the like are irradiated with a pulsed laser is used.
On the other hand, in an EUV radiation producing device of the DPP type, EUV radiation emitted from a high density and high temperature plasma which is produced by power current driving is used. As the discharge methods in an EUV radiation producing device of the DPP type, a Z pinch method, a capillary discharge method, a dense plasma focus method, a hollow cathode triggered Z pinch method and the like are known. The EUV radiation producing device of the DPP type compared to the EUV radiation producing device of the LPP type has the advantages of a downsized radiation source device and a less power consumption of the radiation source system. Therefore, the EUV radiation producing device of the DPP type is strongly expected for practiced use in the market.
In the above described EUV radiation producing devices of the two types. Roughly decavalent xenon (Xe) ions and tin (Sn) ions are currently considered as the radiating species which emits EUV radiation with a wavelength of 13.5 nm, i.e., the raw material for the high density and high temperature plasma. Especially, tin has several times higher EUV conversion efficiency than that of xenon. Here, EUV conversion efficiency is defined as the ratio of the EUV radiation output with a wavelength of 13.5 nm to the electrical input for producing a high density and high temperature plasma. Therefore, tin is most promising radiation species for a high power EUV radiation source of the production model. As, for example, disclosed in Japanese patent disclosure document 2004-279246 (corresponding to U.S. Pat. No. 6,984,941), an EUV radiation source using a gaseous tin compound (for example, stannane gas: SnH4 gas) as the raw material for supply of tin as the EUV radiating species is being developed.
However there is also the disadvantage. Tin is a solid at room temperature and has a low vapor pressure. Therefore, a large amount of debris due to the tin is arising when tin and/or tin compound, which are being fed into a radiation source chamber, is heated and excited, and a high density and high temperature plasma is produced.
The EUV radiation producing device outputs EUV radiation to a subsequent stage, which is emitted by a high density and high temperature plasma which is produced in the radiation source chamber via a collector mirror which is located in the radiation source chamber to a subsequent stage. In the case in which debris due to the tin is deposited on this collector mirror, the reflectivity of the collector mirror is reduced with respect to the wavelength of 13.5 nm. As a result, the output of the EUV radiation to a subsequent stage is reduced.
To overcome this disadvantage, the applicant in Japanese patent application 2005-15182 (corresponding to U.S. patent application No. 11/337539) has proposed a process and a device for implementing this process in which hydrogen radicals react with tin which has deposited on the collector mirror, in which the tin with a low vapor pressure is converted into tin hydride with a high vapor pressure and thus is transferred into the gaseous state, and in which the tin which has deposited on the collector mirror is removed (the collector mirror is cleaned). In this connection, the term “hydrogen radicals” means hydrogen atoms (also called atomic hydrogen). They have unpaired electrons, by which the activation effect is high. They are particles with high reactivity. The process for producing the hydrogen radicals can be a thermal catalyst method, a discharge plasma method and the like. In the thermal catalyst method, the hydrogen gas is brought into contact with a metal with a high melting point, such as tungsten or the like, which is heated for example to roughly 1800° C. and dissociates, by which high density radicals are produced. For the discharge plasma method, which are produced in the plasma together with ions, generated by a radio-frequency discharge or a microwave discharge, are used.
The tin hydride which is formed in this cleaning process, and at least part of the raw material which contains the tin supplied to the discharge plasma region, are evacuated from the radiation source chamber to outside of the EUV radiation producing device by an evacuation device connected to the radiation source chamber. The evacuation device comprises, for example, a vacuum pump, such as a turbomolecular pump or a rotary pump which is connected downstream of the turbomolecular pump or the like. In the gas which is released from the radiation source chamber, tin, tin hydride and tin compounds are contained. The tin, which exists in the evacuated gas and which is formed by decomposition of tin hydride and of a tin compound in the evacuated gas, adheres as solid tin to the rotor and the sliding part of the vacuum pump of the evacuation device; this leads to the disadvantage of a decreased of the evacuation performance of the evacuation device or to similar disadvantages. Therefore, there is the disadvantage that, in an EUV radiation producing device using tin as the EUV radiating species, the period of the maintenance and the replacement period of the evacuation device of the EUV radiation producing device must be shorter than in an EUV radiation source using xenon as the EUV radiating species.
In the field of semiconductor manufacture in which the EUV radiation producing device is practically used, the mean time to repair (MTTR) is regarded as an important parameter with respect to production costs. Since increase of the shutdown of the device due to maintenance, i.e., an increase in the mean time to repair, leads directly to an increase of production costs, there is a demand for reducing the shutdown of the device to a minimum. Therefore, it is required that the maintenance of the evacuation device in the EUV radiation producing device can be performed while the EUV radiation producing device is being operated, or an evacuation device with a long service life is expected.